The present invention relates to microwave filters and in particular to a comb-line bandpass filter of narrow bandwidth with a low insertion loss.
Prior art comb-line bandpass filter of narrow bandwidth for use in the microwave region comprises a plurality of inner conductors successively arranged in an enclosure or outer conductor, each inner conductor being conductively coupled to the outer conductor at one end thereof and capacitively coupled thereto at the other end. In order to provide a low-loss bandpass filter, it is necessary to provide a high unloaded Q value. This in turn requires that each inner conductor have a relatively greater axial length. However, for a given value of coupling between inner conductors, an increase in axial length of each inner conductor will result in a design in which they are spaced an increased distance from each other to compensate for the increase in coupling, so that the overall size of the bandpass filter is considerable.